Significant improvements in the switching voltage distribution are required for the development of unipolar resistivememory devices using MnOx thin films. The Vset of the as-grown MnOx film ranged from 1 to 6.2 V, whereas the Vset of theoxygen-annealed film ranged from 2.3 to 3 V. An excess of oxygen in an MnOx film leads to an increase in Mn4+ contentat the MnOx film surface with a subsequent change in the Mn4+/Mn3+ ratio at the surface. This was attributed to the changein Mn4+/Mn3+ ratios at the MnOx surface and to grain growth. Oxygen annealing is a possible solution for improving theswitching voltage distribution of MnOx thin films. In addition, crystalline MnOx can help stabilize the Vset and Vreset distributionin memory switching in a Ti/MnOx/Pt structure. The improved uniformity was attributed not only to the change of thecrystallinity but also to the redox reaction at the interface between Ti and MnOx.